1. Field of the Invention
This invention relates to a focusing apparatus for bringing an imaging optical system and a surface to be inspected into an in-focus state, and is suitable for use in a microscope.
2. Related Background Art
A focusing apparatus for use in a microscope of the episcopic-illumination type is disclosed, for example, in U.S. Pat. No. 3,721,827.
This apparatus, as shown in FIG. 11 of the accompanying drawings, has a slit light source comprising a light source 101 for measurement, a condensing lens 102 and a slit plate 103. Of the optical paths of light beams from the slit plate 103, one optical path 111 bisected by a plane containing the optical axis of the condensing lens 102, is intercepted by a light intercepting plate 104. Of the light beams from the slit plate, a light beam passed through the other optical path 112 is reflected by a half mirror 105, passes through an optical path 113 at the left of the optical axis of an objective lens 106 to the objective lens 106, and is imaged on a surface 107 to be inspected. The reflected beam of light from the surface 107 to be inspected passes through an optical path 114 at the right of the optical axis and through the objective lens 106, and is re-imaged on the light receiving surface of a light receiving element 108. The light receiving surface of the light receiving element 108 is bisected by the optical axis of the objective lens 106, and outputs are obtained from two light receiving areas 108a and 108b, respectively.
When the positional relation between the objective lens 106 and the surface 107 to be inspected is in an in-focus state, the reflected beam of light from the surface 107 to be inspected is re-imaged on the boundary line between the light receiving area 108a and the light receiving area 108b. When the positional relation between the objective lens 106 and the surface 107 to be inspected is farther than the in-focus state, the reflected beam of light from the surface 107 to be inspected is re-imaged on this side of the light receiving surface of the light receiving element 108 (front focus state) and enters the light receiving area 108a. When the positional relation between the objective lens 106 and the surface 107 to be inspected is closer than the in-focus state (rear focus state), the re-imaged position of the reflected beam of light from the surface 107 to be inspected is rearward of the light receiving element 108, and on the light receiving surface, the reflected beam of light enters the light receiving area 108b.
It is possible to judge that the focus state is the front focus state when the differential signal between the outputs of the light receiving area 108a and the light receiving area 108b is positive, that the focus state is the rear focus state when said differential signal is negative and that the focus state is the in-focus state when the differential signal is zero. It is possible to judge the position of the surface to be inspected from this differential signal and to automatically effect alignment.
In recent years, due to the spread of liquid crystal displays, etc., the inspection of a liquid crystal pattern formed on a transparent substrate such as glass is becoming an important industrial task, and it is popular to use a microscope for such inspection.
However, when the inspection of a transparent substrate like a liquid crystal substrate is effected by the use of the apparatus as shown in FIG. 11, an accurate in-focus state is not obtained. That is, glass is a substance of low reflectance and the intensity of the reflected beam of light from the surface of an object to be inspected and the intensity of the reflected beam of light from the back of the object to be inspected become nearly equal to each other. When the positional relation between the objective lens and the surface of the object to be inspected is in the in-focus state, the re-imaged position of the reflected beam of light from the surface of the object to be inspected is on the light receiving surface of the light receiving element. However, the reflected beam of light from the back of the object to be inspected is re-imaged on this side of the light receiving surface of the light receiving element and enters the light receiving area 108a on the light receiving surface. Therefore, in spite of being focused on the surface to be inspected, a differential signal representative of the not in-focus state is outputted from the light receiving element, and the focusing apparatus determines that the focus state is not the in-focus state.